1. Field of the Invention
The present disclosure is directed to a wellbore survey method and apparatus, and more particularly to a survey system which enables mapping of the well borehole path while moving a survey instrument along the well borehole during drilling
2. Background of the Art
Well borehole survey can be defined as the mapping of the path of a borehole with respect to a set of fixed, known coordinates A survey is required during the drilling of many oil and gas wells, and is of particular importance in the drilling of well which is deviated significantly from an axis perpendicular to the earth surface. Often two or three surveys will be required during the drilling process. In addition, a final survey is often required in a highly deviated well.
In drilling an oil well, it is rather easy to drill straight into the earth in a direction which is more or less vertical with respect to the surface of the earth. Indeed, regulatory agencies define a vertical well by tolerating a few degrees of deviation from the vertical. The interruption of the drilling operation and cost of the surveys is minimal in that situation. By contrast, highly deviated wells are required in a number of circumstances.
Onshore, it is necessary to drill a deviated well to enter formations at selected locations and angles. This may occur because of the faulting in the region. It is also necessary to do this around certain types of salt dome structures. As a further example of onshore, deviated drilling, a tremendous amount of interest has been developed in providing surveys of wells that have been deviated from a vertical portion toward the horizontal. Recently, a number of older wells drilled into the Austin chalk formation in the south central United States have played out and production has been lost. This has been a result of the loss of formation pressure. The Austin chalk producing strata is easily located and easily defined. It is however relatively thin. Enhanced production from the Austin chalk has been obtained by reentering old wells, milling a window in the casing, and reentry into the formation. The formation is typically reentered by directing the deviated well so that it is caught within the producing strata. In instances where the strata is perfectly horizontal with respect to the earth, that would require horizontal hole portion after curving into the strata. As a practical matter, the producing formations may also dip and so the last leg of the well may extend outwardly at some extreme angle such as 40 to 70xc2x0. Without being definitive as to the particular formation dip, such drilling is generally labeled horizontal drilling. The end result is that the borehole does not simply penetrate the formation, but is directed or guided follow the formation so that several hundred feet of perforations can then be placed to enable better production. To consider a single example, assume that the formation is 20xe2x80x2 thick measured from the top to the bottom face. Assume as an example that the formation has a dip of 30xc2x0. By proper direction of the well during drilling, several hundred feet of hole can be drilled between the top and bottom faces of the formation. After drilling, but before casing has been completed, it is often necessary to conduct a concluding survey to assure that the production is obtained below the leasehold property. In addition, other surveys are required.
In offshore production, once a producing formation has been located, it is typically produced from a centrally positioned platform. Assume that the producing formation has an extent of four or five miles in lateral directions. Assume further that the formation is located at 5,000 feet or deeper. A single production platform is typically installed at a central location above the formation and supported on the ocean bottom. A production platform supports a drilling rig which is moved from place to place on the platform so that a number of wells are drilled. It is not uncommon to drill as many as 32 or more wells from a single production platform. From the inception, all the wells are parallel and extend downwardly with parallel portions, at least to a certain depth. Then, they are deviated at some angle. At the outer end of the deviated portion, vertical drilling may again be resumed. While a few of the wells will be more or less vertically drilled, many of the wells will be drilled with three portions, a shallow vertical portion, an angled portion, and a termination portion in the formation which is more or less vertically positioned. Again as before, one or two surveys are required during drilling, and a completion survey is typically required to be able to identify clearly the location of the well in the formation. Field development requires knowledge of the formation itself and also requires knowledge of the termination points of the wells into the formation. This means accurate and precise surveys are used to direct the wells in an optimum fashion to selected locations to get proper production from the formation.
The use of magnetic survey instrumentation is widely applied, but this technology has its limitations. For example, locally, magnetic survey instrumentation accuracy can be limited, since the earth""s magnetic field strength and dip angles change, causing erroneous magnetic survey readings. Furthermore, magnetic survey accuracy can also be distorted due to non magnetic drill collars or so called xe2x80x9chot-spotsxe2x80x9d. In addition, the magnetic survey accuracy can also be negatively affected by the presence of adjacent wells, from which the steel casing may severely influence the earth""s magnetic field thereby generating erroneous magnetic readings within the well being surveyed. Other issues which affect the magnetic survey accuracy are the platform mass from which the survey is being conducted, geomagnetic interferences, and changes in the earth""s magnetic field from one location to another location. Of course, these changes can be accurately measured, but in practice it is not a routine procedure and it further requires well trained field engineers and sophisticated instrumentation. Magnetic survey technology is also not applicable for use in wellbore which have been cased with steel casing.
The mapping apparatus, containing a rate gyroscope and accelerometers, remotely measures the earth""s spin axis, and is lowered into the wellbore, while the system is held stationary at predetermined locations. In addition, the apparatus applies a rotary drive mechanism, functionally connected with the gyroscope and the accelerometers to rotate the gyroscope about its instrument or housing axis. Furthermore, the mapping apparatus contains a downhole power supply and data section for processing the sensor outputs to determine the heading direction of the wellbore at predetermined wellbore depths. This invention also discloses a method to measure azimuth very accurately regardless the wellbore deviation angle and latitude, while traversing continuously through a wellbore. A major advantage over U.S. Pat. No. 4,611,405 is the absence of a feed back controlled mechanism, i.e. the absence of a resolver means which is connected with a drive mechanism. In addition, the absence of a costly, power consuming feed back controlled mechanism reduces, significantly, development, operation and maintenance costs.
Survey instruments introduced in the 1980""s featured rate gyroscopes and inclinometers in various configurations have been used for a number of years. A representative survey system of that sort is shown in U.S. Pat. No. 4,468,863 and also in U.S. Pat. No. 4,611,405. These instruments do not utilize a measure of the earth""s magnetic filed, and can therefore be used in cased boreholes, and further overcome other previously discussed shortcomings of magnetic surveys. In these systems, a gyroscope is mounted with an axis of rotation coincident with the tool body or housing. The housing is an elongate cylindrical structure. Accordingly, the long housing is coincident with the axis of the well. That type system additionally utilizes X and Y axis accelerometers which define a plane which is transverse to the tool body thereby giving instrument inclination and orientation within the borehole. As the well deviates from the vertical, the axis of the gyroscope then is pointed in the correct azimuthal direction. By reading gyroscope movement, the azimuth can be determined and, when combined with the accelerometer measurements, the path of the borehole can be mapped in space.
In present day onshore and offshore drilling operations, highly deviated boreholes being drilled for reasons outlined above. High angles of deviation from the vertical often result in a rather small radius of curvature, or sharp bend in the borehole, thereby limiting the length and diameter of survey equipment that can traverse these bends. The prior art gyro/accelerometer systems discussed above, which are still widely used today, range in diameter up to 10⅝ inches and in length up to 40 feet. These dimensions introduce severe operational problems in traversing sharp or xe2x80x9ctightxe2x80x9d bends in today""s highly deviated wells.
The prior art gyro/accelerometer systems are quite complex and expensive to fabricate and to operate. Still further, these systems must be stopped at discrete survey locations or xe2x80x9cstationsxe2x80x9d within the borehole to obtain xe2x80x9cpointxe2x80x9d readings. The survey instrument is stopped to permit a servo drive control system to restore one of the accelerometers to the horizontal. In effect, the gimbal or other support mechanism for the survey instrument is driven until the accelerometer is positioned in a horizontal plane. There are rather difficult calculations required to recognize the horizontal reference planes sought in that instance. The servo loop must be operated to seek that null position. Once that position is obtained, readings can be taken. This however requires stopping the equipment and permitting an interval of time while the servo loop accomplishes nulling. This requires taking a data point only at specified locations, so that a continuous curve representative of the borehole survey is merely an extrapolation of a number of discrete data points which are taken in space and which are formed into a curve utilizing certain averaging procedures. Furthermore, multiple stationary measurements greatly increases the cost of the survey in increased drilling rig time.
An object of the present invention is to provide a wellbore survey system which will operate in both open boreholes and boreholes cased with steel casing.
Yet another object of the invention is to provide accurate survey data over a wide range of borehole deviation ranging from essentially vertical boreholes to boreholes deviated from the vertical to angles of 90 degrees or more.
A further object of the invention is to provide a borehole survey system which can be conveyed along a wellbore and yield continuous borehole survey data without accuracy degradation in conjunction with quantifiable survey precision.
A still further object of the invention is to provide a survey instrument which is relatively short in length to negotiate short radius curves within the borehole.
Another object of the invention is to provide a smaller diameter survey instrument which can be pumped down the borehole.
Further objects of the invention are to provide a survey instrument which is rugged, reliable, relatively inexpensive to manufacture and operate, and which can be operated at relatively high temperatures.
Another object of the invention is to provide an embodiment which can be mounted in a drill collar and which will provide a map of the borehole obtained during the drilling of the borehole. Measurements obtained during the drilling operation are commonly referred to as measurements-while-drilling or simply xe2x80x9cMWDxe2x80x9d.
Yet another object of the invention is to provide a system which yields MWD measurements of borehole azimuth and inclination each time the drill string is stopped to add another string of drill pipe, wherein each measurement is initiated by a down hole vibration sensor which activates the system when vibration ceases thereby indicating that the drilling has ceased.
There are other objects of the invention which will become apparent in the following disclosure.
The present disclosure provides a markedly improved wellbore survey system. The downhole survey instrument or xe2x80x9cprobexe2x80x9d utilizes a set of accelerometers which are mounted in the probe""s cross borehole plane and mutually perpendicular to one another. In addition, the probe utilizes a dual-axis rate gyroscope, with its spin axis aligned with the axis of the probe. Two measurement principles, the gyrocompassing technique and the continuous survey mode, are employed to calculate wellbore direction as a function of depth. Both principles, and their application to the desired measurement, will be briefly summarized.
The gyrocompassing survey technique is employed to survey near vertical wellbore sections, and to measure the initial heading reference prior to switching to the continuous mode. During the gyrocompassing procedure, the probe is lowered into the wellbore by means of an electric wireline to measure the earth""s gravity field and the earth""s rate of rotation while the probe is held stationary at predetermined depths. The accelerometers measure the earth""s gravity field. This allows computation of the instrument roll angle by determining the ratio of the output of the x-axis accelerometer over the output of the y-axis accelerometer. In addition, mathematical projection of the output of the x-axis accelerometer and the output of the y-axis accelerometer onto the highside direction enables computing the wellbore deviation angle. The azimuth angle is invariant to the earth""s gravity field and therefore an additional sensor is used to determine the azimuth angle of the wellbore deviation angle. This is provided by the gyro readings as described in the following paragraph. The rate gyro sensor measures the earth""s rate of rotation. Since the earth rotates at a fixed speed and these measurements are made at a given latitude, the vertical and horizontal earth rate vector components can also be derived. These components can then be projected into the sensitive gyro axis plane where the horizontal earth rate component references true north. The rate gyro, therefore, provides an azimuth reading referenced to a fixed point such as true north. By combining the output of the gyro sensitive axes and the accelerometer outputs, the well bore direction, inclination, and tool face can be determined. Depth is incorporated from the amount of wireline deployed to lower the probe within the borehole. Combining a series of survey stations downhole through a calculation method such as minimum curvature yields wellbore trajectory.
The continuous survey mode is based on measuring relative instrument rotations while the probe is continuously traversing through the borehole. After taking a stationary reference heading measurement in the gyrocompassing mode, new modeling procedures allow computation of probe azimuth and inclination changes about the highside and highside right directions, where the highside right direction is at right angles with respect to the highside direction. This is accomplished by mathematically projecting the probe azimuth and inclination changes into the gyro sensitive axis plane.
In order to calculate the actual wellbore path, the rate of rotation about the highside and highside right are integrated over time, yielding wellbore heading and inclination changes from the previously described reference procedure. In conjunction with depth, which is derived by continuously monitoring the amount of wireline deployed, the wellbore trajectory is generated.
An important advantage of the continuous mode is that, unlike gyrocompass surveying, continuous operation has no limitations in angle of inclination above 10 to 15 degrees.
Another obvious advantage of the continuous mode of operation is that the stopping and starting, and the time required to make station measurements, are avoided. Consider as an example that a survey of a well that has a length of 10,000 feet is required. Using the prior art station measurement technique, measurements should be taken at intervals not exceeding 100 feet. Using this criterion, one hundred measurements are required, wherein each measurement requires approximately one minute. Even if the top ten or twenty measurements are skipped because the top portion is fairly well known to be vertical, eighty to ninety station measurements are still needed. If the continuous mode survey of the present invention can eliminate eighty to ninety station measurements, a significant amount of time can be saved. Although time is required to establish a reference heading, and the continuous survey mode does require a finite amount of time, it is estimated that use of the present invention would result in a 25 to 50% reduction in interruption in the drilling process to obtain the survey. If one hour is saved per trip, rig time is reduced by one hour, and on land, that can have a value of easily $500.00 or more per hour. In an offshore drilling vessel, one hour of rig time may cost as much as $5,000-$10,000 per hour. Prices may vary up or down. It is therefore extremely beneficial to be able to run a survey without having to start and stop time and time again.
Another advantage of the present invention is that the quality of the data obtained from the survey is improved by a great amount over station measure surveys, in that measurements made in the continuous mode provide a continuous curve of the measurements. This then enables integration over the time interval of the survey. This permits a continuous survey to be provided. The present survey method and apparatus are probably more accurate than a survey furnished with discrete, stationary data points.
The present invention yields survey data which is not adversely affected by the angle of wellbore inclination. Furthermore, the probe of the present invention is relatively small in diameter, short in length, and can be reliably operated at relatively high temperatures.
In an alternate embodiment, the survey apparatus can be mounted in a drill collar in order to map the path of the borehole during the borehole drilling operation. Measurement obtained during the drilling operation are commonly referred to as measurements-while-drilling or simply xe2x80x9cMWDxe2x80x9d. In the MWD embodiment, the survey apparatus is conveyed by the drill string rather than a wireline. Furthermore, directional measurements are made each time the drill string is stopped to add typically a thirty foot length of drill pipe. This yields xe2x80x9cstationxe2x80x9d measurements of borehole azimuth and inclination every thirty feet thereby mapping the path of the borehole as the borehole is advanced. Alternately, the survey system can be equipped with a third or z-axis accelerometer to enhance the inclination measurements in highly deviated boreholes. During drill string rotation, vibrations at the drill collar are quite intense. A vibration sensor mounted within the drill collar is used to determine, downhole, whether the drill string is advancing the borehole or whether drilling has ceased. Upon sensing that drilling has ceased, the vibration sensor automatically activates the survey system, and directional parameters are measured. Measurement is automatically terminated when drilling is again resumed, and the measured directional information is stored within a downhole memory device and identified by the borehole xe2x80x9cstationxe2x80x9d at which the information was obtained. This process is repeated as lengths or xe2x80x9csectionsxe2x80x9d of drill pipe are added to advance the borehole. When the drill string is removed or xe2x80x9ctrippedxe2x80x9d from the borehole in order to replace the drill bit, or for other reasons, directional data are retrieved from the downhole memory and processed as a function of measure positions within the borehole to yield a map of the borehole in three dimensional space.
In summary, the present disclosure sets out a survey method and apparatus which utilizes a rate gyro having a spin axis coincident with the shell or housing of the downhole instrument probe, which in turn is coincident with the axis of the well borehole. Two accelerometers positioned at right angles are mounted to define a transverse plane at right angles across the instrument. Alternately, a third accelerometer can be employed with an axis parallel to the major axis of the instrument. The probe housing is permitted to tumble or rotate in space in the continuous survey mode so that continuous movement including rotation of a random amount and direction is permitted. The output obtained from the system is a continuous data flow, i.e., a continuous well survey can then be obtained. In an alternate MWD embodiment, the survey instrument yields directional data at each point within the borehole at which drilling is stopped to add a section of drill pipe.